Spin-polarized tunneling in room-temperature mesoscopic spin valves

APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 24 13 DECEMBER 2004Spin-polarized tunneling in room-temperature mesoscopic spin valvesS. O. Valenzuela a) and M. TinkhamGordon McKay Laboratory, Harvard University, Cambridge, Massachusetts 02138(Received 6 July 2004; accepted 12 October 2004)We study optimization of spin injection and detection both at 4.2 K and at room temperature usinga metallic mesoscopic spin valve structure with tunneling interfaces between the ferromagneticelectrodes (CoFe or NiFe) and the central metallic conductor (Al). We investigate the influence ofthe barrier transparency on the spin polarization of the tunneling electrons by varying the O 2exposure of the Al film before depositing the ferromagnetic electrodes. An increase of thepolarization from 10% to 16% (25% at 4.2 K) is observed as the resistance of the junction isincreased from 100 to 700 m 2 . A spin transresistance as high as 2.5 is obtained at 4.2 K. ©2004 American Institute of Physics. [DOI: 10.1063/1.1830685]In recent years, there has been increased interest in magnetoelectronicdevices, which use the spin rather than thecharge of the electron. 1 In these “spintronic” devices, it isdesirable to be able to inject spin currents, manipulate thespin information that they carry and then detect the resultingspin polarization. Spin injection and detection was first demonstratedby Johnson and Silsbee 2 in an aluminum crystal fortemperatures below 77 K. It took more than a decade toachieve electrical spin injection and detection at room temperature.Jedema et al. 3 demonstrated the effect in mesoscopicmetallic spin-valve devices with tunnel–barrier interfaces.The reduced size of these devices helped to increasethe detected signal by six orders of magnitude as comparedto the seminal experiments of Johnson and Silsbee. However,the estimated polarization of the tunneling electrons P10% (for Co electrodes) is much lower than previouslyreported values 4 which are as high as 40%. This reduction inthe polarization is not completely understood but could berelated to the larger transparency of the tunnel Al 2 O 3 barriersused in the spin valve experiments. The thickness of theAl 2 O 3 barrier in this case is estimated to be only 1 nmascompared to 2 nm in the experiments of Ref. 4.In this letter, we investigate the influence of the barriertransparency on the spin polarization of the tunneling electrons.We observe an increase of the effective polarization atroom temperature from around 10% to 16% (25% at 4.2 K)as the oxygen pressure in the oxidation process is increasedfrom 10 to 150 mTorr. This increment in the polarizationand the reduction of sample dimensions (i.e., the Al thicknessand the distance between the electrodes) further enhancedthe output signal of our devices by another two ordersof magnitude as compared to previous work in similardevices 3 and an order of magnitude as compared to morerecent measurements in a mesoscopic Al island. 5 In the optimizeddevices, we obtain spin transresistances as high as2.5 . Finally, we cooled the sample down to 0.25 K andcompared the polarization values obtained from the spinvalveeffect with the conventional method of using theZeeman-split states in superconducting Al as spin detector.We prepare the devices (Fig. 1) with electron beam lithographyand a three-angle shadow evaporation techniqueto produce tunnel barriers in situ (without breaking vacuum).a) Electronic mail: sov@rsj.harvard.eduThe shadow mask is made with a standard PMMA-MMAbilayer. The base resist (MMA) has a sensitivity 5 timeslarger than the top resist (PMMA) which allows to makesuspended masks with large undercuts by selectively exposingthe bilayer resist to different e-beam doses. An aluminumstrip (100–150 nm wide) is first deposited at normal incidencethrough the suspended mask onto a Si/SiO 2 substrateusing e-beam evaporation with a base pressure lower than10 −8 Torr. Next, the aluminum is oxidized in pure oxygen(10–150 mTorr for 40 min) to generate the insulating Al 2 O 3barriers. After the vacuum is recovered, the FM electrodes(60–80 nm wide) are deposited sequentially from two differentangles (±45° from the normal to the substrate surface).For the FM electrodes, we used CoFe (80 wt. % Co) andNiFe (80 wt. % Ni) which have large polarizations and differentcoercive fields.FIG. 1. (a) Atomic force microscope image of a device and schematic of themeasuring configuration. The thin aluminum strip (6–10 nm thick) is oxidizedand contacted with two ferromagnetic (FM) electrodes of differentthicknesses (CoFe, 20 nm; NiFe, 35 nm); (b) and (c) scanning electron microscopeimages of two devices with different distances between the FMelectrodes. Bars are 200 nm long.0003-6951/2004/85(24)/5914/3/$22.00 © 2004 American Institute of Physics5914Downloaded 13 Sep 2006 to 18.140.1.140. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Appl. Phys. Lett., Vol. 85, No. 24, 13 December 2004 S. O. Valenzuela and M. Tinkham 5915Tunnel junctions form at the overlapping regions withthe Al strip with an area resistance in the range of100–700 m 2 depending on the oxygen exposure.Samples made under the same conditions typically presenteda variation below 20% in the resistance of the tunnel junctions.The thickness of the NiFe and CoFe electrodes werechosen to be 20 and 35 nm, respectively to geometricallyenhance the difference in their coercive fields. A small magneticfield of 60 Oe along the FM electrodes was appliedduring the growth to favor the alignment of their magnetocrystallineand shape anisotropies.We made several sets of four samples; each of them in asingle deposition with different oxidation conditions and differentAl thicknesses. The distance d between the FM electrodes(center to center) was varied from 150 to 700 nm. Theactual dimensions of the samples were measured with a scanningelectron microscope (SEM). The estimated conductivityof the Al, Al , for various samples was found to be in therange 0.3–1.110 7 −1 m −1 and increased with the thicknessof the Al film 6–10 nm. The samples were classifiedas a function of the tunnel junction resistances. We discardedsamples showing an anomalously different resistance in anyof the two junctions as compared to the rest of the samples intheir corresponding batch.In our experiments, we source spin-polarized electronsinto the aluminum (Al) strip from one of the FM electrodes[CoFe in Fig. 1(a)]. A current I from or into the source resultsin an unequal density of spin-up and spin-down electronsin the aluminum 2,3 with a difference proportional to theeffective polarization of the CoFe electrode, P CoFe , whichcharacterizes the difference between the spin-up and spindownpopulations of the electrons that participate in the tunneling.For low bias, P CoFe can be written as P CoFe =N ↑−N ↓ /N ↑ +N ↓ where N ↑,↓ are the electronic spin-up (↑) andspin-down (↓) density of states at the Fermi level of thetunneling electrons in the FM (CoFe).The spin imbalance created by the current will diffuse inboth directions along the Al strip and will reach the detectorelectrode (NiFe) which will measure an average of the twospin densities weighted by its own polarization, P NiFe . Therefore,the output voltage V is sensitive to the spin degree offreedom 2,3 and is proportional to P CoFe P NiFe . More explicitly,the magnitude of the output transresistance V/I is equalto: 2,3 V ±I =±1 2 P sfCoFeP NiFe Al A e−d/ sf,1where sf is the spin relaxation length and Al and A are theconductivity and cross sectional area of the Al strip. Here the() and () signs correspond to parallel and antiparallelconfigurations of the electrodes magnetizations. The spin relaxationlength can be obtained by fitting the transresistanceas a function of d to Eq. (1). The geometric mean P of thepolarization of the electrodes P= PCoFe P NiFe is then calculatedusing sf . It is worth noting that we measured severalsamples each with a single pair of FM electrodes with adefinite spacing d as opposed to one sample with multipleelectrodes in order to avoid possible anomalous spin-flipscattering at the interfaces with each FM.The measurements were performed using lock-in techniquesby applying a current I1 A through the sourcejunction and measuring ac voltage at the remote detectorFIG. 2. (a) Spin-valve transresistance V/I for two samples: one at 4.2 K(larger variation) and the other at room temperature (RT), I=1 A; (b)logarithm of the spin transresistance change R=V/I as a function of thedistance d between the ferromagnetic electrodes for four sets of samples.The top curve (open circles) was taken at 4.2 K the rest at RT. The thicknessof the Al strip is 10 nm (triangles) and 6 nm (open and solid circles anddiamonds). The lines are fittings to Eq. (1). Inset: R as a function ofcurrent bias for a sample with d=215 nm at T=4.2 K. A significant drop inR is seen for I1 A.junction as sketched in Fig. 1(a). The results are independentof which electrode is used as source or detector. For largerbiases, we found that the effective polarization drops 6 [seeinset of Fig. 2(b)]. In this letter, we concentrate on the lowbias response.Figure 2(a) shows typical spin-valve transresistance V/Imeasurements at 4.2 K and at room temperature as a functionof an applied in-plane field along the axis of the FMelectrodes. At large negative magnetic field we have a parallelconfiguration of the electrodes magnetization and R=V + /I. As the magnetic field is swept from negative to positive,a change in the sign of the detector signal is observedwhen the magnetization of the NiFe electrode reverses andthe device switches from parallel to antiparallel configurationR=V − /I=−V + /I. As the magnetic field is further increasedthe CoFe flips and the detector signal changes sign again asa parallel configuration is recovered.Figure 2(b) shows the change in output spin transresistanceR=V + −V − /I as a function of d for different sets ofsamples. By fitting the data to Eq. (1), we calculate sf andP. The product Al A is obtained independently by measuringd (with a SEM) and the resistance between the FM electrodes,R Al A=d/R. Our measurements show transresistancesas high as 2.5 4.2 K for the samples with thelargest effective polarization (25%) and with the smallest d.The polarization drops to around 16%–17% at room temperature.Although no reduction in P was observed in Ref. 3,a similar reduction has been found in the magnetoresistancein metallic magnetic tunnel junctions which was ascribed toa loss of magnetization at large temperatures due to magnonexcitations at the FM-barrier interface 7 and thermally assistedspin-independent tunneling. 8It is interesting to note that for the thin Al strip 6 nmthe slope of the lines in Fig. 2(b) does not change with temperatureand, as expected, it is independent of the barriertransparency. In all cases, the fitted spin relaxation length, sf , is around 200 nm. Measurements performed on thickerAl strips 10 nm, open triangles in Fig. 2(b), presented alarger sf 420 nm suggesting that the spin-flip in the thinnersamples is dominated by surface scattering.More important, we also found a systematic increase inthe spin polarization as the barrier transparency decreased(higher O 2 pressure in the barrier preparation). This is clearlyDownloaded 13 Sep 2006 to 18.140.1.140. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

5916 Appl. Phys. Lett., Vol. 85, No. 24, 13 December 2004 S. O. Valenzuela and M. TinkhamFIG. 3. (a) Calculated polarization P by fitting the spin-valve transresistancedependence on the distance between the ferromagnetic electrodes to Eq. (1).Lowest resistance polarization value is from Ref. 3. The line is a guide to theeye: (b) and (c) experimental dI/dV curves at 0.25 K for 0 T (open circles)and 2 T (solid circles). NiFe (b); CoFe (c). These are four probe measurementswith a 4 V, 42.2 Hz sine voltage.represented in Fig. 3(a) and it could be relevant to understandwhy the polarization reported in Ref. 3 is lower than inprevious studies. 5 It is generally accepted that the spin polarizationis dominated by sp-like electrons because of theirlarge contribution to the tunneling due to their highly delocalizednature 9,10 (as compared to d electrons). However, areduction of the effective tunneling polarization could be expectedif thinner barriers lead to an increased contribution tothe tunneling of the highly localized d states (see, for example,Refs. 11 and 12). Pinholes in the barriers and achange in the barrier height 13 could also lead to a reductionin the polarization. Pinholes, however, are unlikely to be responsibleas no leakage current is observed in our conductancemeasurements at low temperature 12 [see below andFigs. 3(b) and 3(c)]. The general behavior of spin polarizedfree-electron models should be analyzed as a function of thetunnel-barrier properties. Experimentally, a systematic studyof the bias dependence of the polarization for different oxidationtimes would provide valuable information on the natureof the tunneling electrons.Finally, we used the traditional Meservey-Tedrowtechnique 4 to readily measure the tunneling polarization ofour junctions and compare it with the values obtained above.This technique is limited to low temperatures, below the superconductingcritical temperature of the Al strip. The Zeemansplitting in the density of states of the superconductor,induced by a large magnetic field, is used as a spin detector.The orbital depairing in the samples with the thinnest Al isstrongly reduced when the magnetic field is applied in thesubstrate plane. The tunneling junctions were cooled down to0.25 K and the tunneling conductance was measured at zerofield and at 2 T.Figures 3(b) and 3(c) show the differential tunnelingconductance curves for a typical NiFe [(b)] and CoFe [(c)]junction. At H=0, the conductance exhibits the typical goodqualityjunction behavior, presenting no leakage current atzero voltage. Zeeman splitting of the quasiparticle density ofstates is clearly seen at H=2 T. The asymmetry in the conductancecurve in this case is used to extract the value of thepolarization of the ferromagnet. A rough estimate of the spinorbitcorrected polarization can be obtained 4,14 from theheight of the four conductance peaks namely, P CoFe =48%and P NiFe =29%. Their geometrical average is 37% which islarger than the value obtained from Eq. (1) for the samesample (25%). It is worth noting that this rough calculationhas been found 14 to systematically overestimate the actualpolarization by 5%–10% and a more careful fitting of thedata of Figs. 3(b) and 3(c) would be required for more accuratevalues. With a 10% reduction in the above estimatedvalues of P CoFe and P NiFe , P27% is in closer agreementwith the fitting to Eq. (1) at 4.2 K. A detailed analysis and acomparison between the values of P obtained with these experimentsis important and will be reported elsewhere.In summary, we have demonstrated spin injection anddetection in mesoscopic spin valves both at room temperatureand 4.2 K. We found that for low transparency barriersthe polarization of the injected electrons grows. For the lowtransparency barriers, we obtained spin transresistances ashigh as 2.5 .The authors thank John U. Free for a careful reading ofthe manuscript. This research was supported in part by NSFGrant Nos. DMR-0244441 and NSECPHY-0117795 andONR Grant No. N00014-02-1-0055.1 For a recent review see, I. Žutić, J. Fabian, and S. Das Sarma, Rev. Mod.Phys. 76, 323(2004).2 M. Johnson and R. H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985); Phys.Rev. B 35, 4959 (1987); Phys. Rev. B 37, 5236(1988).3 F. J. Jedema, H. B. Heersche, A. T. Filip, J. J. A. Baselmans, and B. J. vanWees, Nature (London) 416, 713 (2002); F. J. Jedema, M. V. Costache, H.B. 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